In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the field of miniaturization. Typically, these miniaturization techniques involve shortening the wavelength of the exposure light source. Until recently, ultraviolet radiation such as g-lines and i-lines have been used as the exposure light source, but recently, KrF excimer lasers (248 nm) have been introduced, and even ArF excimer lasers (193 nm) are now starting to be used.
One example of a resist material that satisfies the high resolution conditions required to enable reproduction of a pattern of minute dimensions is a chemically amplified resist composition, which includes a base resin that undergoes a change in alkali solubility under the action of acid, and an acid generator that generates acid on exposure, dissolved in an organic solvent.
Chemically amplified positive resist compositions that have been proposed as ideal resist materials for methods in which exposure is conducted using a KrF excimer laser typically employ a polyhydroxystyrene-based resin, in which a portion of the hydroxyl groups have been protected with acid dissociable, dissolution inhibiting groups, as the base resin (see, for example, Japanese Unexamined Patent Application, First Publication No. Hei 4-211258).
Examples of the most commonly used acid dissociable, dissolution inhibiting groups include so-called acetal groups, including chain-like ether groups typified by 1-ethoxyethyl groups, and cyclic ether groups typified by tetrahydropyranyl groups, as well as tertiary alkyl groups typified by tert-butyl groups, and tertiary alkoxycarbonyl groups typified by tert-butoxycarbonyl groups.
On the other hand, in addition to the research being conducted on resist materials to enable ultra-miniaturization, research and development is also being conducted on pattern formation methods, to develop techniques capable of overcoming the resolution limits of resist materials.
An example of one such miniaturization technique that has been recently proposed is the thermal flow process, wherein a resist pattern is formed using normal lithography techniques, and the resist pattern is then subjected to heat treatment to reduce the pattern size (see, for example, Japanese Unexamined Patent Applications, First Publication Nos. 2000-188250 and 2000-356850).
Thermal flow is a method in which following formation of a resist pattern using photolithography techniques, the resist pattern is heated and softened, causing the pattern to flow towards the gaps in the pattern, thus reducing the resist pattern size, that is, the size of the portions where the resist is not formed (such as the hole diameter in a hole pattern, or the space width in a line and space (L&S) pattern).
In recent years, with the rate of miniaturization accelerating, further improvements in the resolution of resist materials are now being demanded.
However, the conventional polyhydroxystyrene-based resins described above can no longer be claimed to offer adequate resolution. Furthermore, in terms of the cross-sectional shape of the resist pattern, the degree of rectangularity is also inferior, with problems including poor verticalness of the resist pattern side walls, and rounding of the shape at the top portions of resist patterns such as line and space (L&S) patterns. These problems of rectangularity become particularly significant in thermal flow processes, where the resist pattern is heated and caused to flow following developing.
Furthermore, having investigated improvements in resolution using a wide variety of different materials, the inventors of the present invention made the following discovery. Namely, in those cases where a copolymer containing structural units derived from hydroxystyrene and structural units derived from a (meth)acrylate ester having an alcoholic hydroxyl group, wherein a portion of the hydroxyl groups have been protected with acid dissociable, dissolution inhibiting groups, is used as the resin component of a resist material, raising the protection ratio provided by the acid dissociable, dissolution inhibiting groups improves the resolution. However, raising this protection ratio tends to increase the occurrence of developing defects such as developing residues. The occurrence of developing defects is a disadvantage that can become a significant problem in the production of semiconductor elements.